The present invention relates to compounds, pharmaceutical compositions and methods for the treatment of bacteremia and septicemia and, more particularly, to novel conjugates of bacterial outer membrane binding peptides, preferably having bacterial sensitization activity, and immune cells chemotactic peptides, and pharmaceutical compositions containing same useful in the treatment of bacteremia and/or septicemia following infection by gram negative bacteria, administered alone or in combination with conventional antibiotics.
Blood infection caused by gram-negative bacteria is one of the major challenges facing modern medicine, despite treatment availability with conventional antibiotics (Young, 1985). Mortality rates in the range of 25-40% for gram-negative septicemia have been reported in some medical centers. Antibiotic treatment is often administered at late stages of the disease, usually when symptoms appear. The time required by the antibiotic to kill the pathogen is often too long, thus failing to prevent irreversible tissue damage. Moreover, in many cases the antibiotic is given before performing any sensitivity tests. The emergence of strains resistant to the conventional antibiotics and lack of rapid diagnosis and antibiotic sensitivity pattern of the infecting bacteria are probably among the major causes of the inadequate conventional therapy and high mortality (Cassel, 1995; Chin & Marks, 1994).
Polymyxins:
Polymyxins are basic cyclic peptides having a lipid moiety and antibiotic activity, naturally produced by various strains of Bacillus polymyxa. Of the many isolated and characterized polymyxins, only polymyxin B and E (the latter is also known as colistin) were in actual therapeutic use (Lambert and O'grade 1992). Polymyxin B is a polycationic amphipathic decapeptide, a potent bactericidal agent against most gram-negative bacteria. Emergence of resistant strains to polymyxin B has been reported to be rare (Soogard, 1982). Polymyxin B binds to the outer membrane of the bacteria (Morrison and Jacobs, 1976), and inserts its lipid moiety into the membrane to completely disorganize it (Bryan, 1982). The clinical use of polymyxins has been restricted, however, because polymyxins are highly toxic to animal cells.
Polymyxin Derived Peptides:
The removal, by deacylation, of the fatty-acid side chain from polymyxins, e.g., polymyxin B and E, significantly reduces the toxicity of the parent molecules (Vaara 1992). The deacylated product of polymyxin B, which is known as polymyxin B-derived peptide (PMBP) and alternatively as polymyxin B nonapeptide (PMBN), lacks bactericidal activity but retains its ability to bind specifically to the lipopolysaccharide (LPS) of the outer leaflet of the bacterial membrane and renders the gram-negative bacteria susceptible to several antibiotics by permeabilizing their outer membrane. The latter antimicrobial activity of PMBP is referred to in the art as sensitizing activity.
Because the parent polymyxin B molecule and its derivative PMBP bind to the same receptor on the bacterial surface, probably via the cyclic heptapeptide portion, the emergence of strains resistant to the sensitizing activity of PMBP is rare, as is the case for the bactericidal activity of the parent polymyxin molecule (Chihara et al., 1973; Vaara and Vaara, 1983; Duwe et al., 1986, Ofek et al., 1994).
Formyl Chemotactic Peptides (fCP):
Schifmann et al., (1975) have shown that chemotaxis (migration of cells toward a gradient of a chemoattractant molecule) of phagocytic cells is induced by short-chain N-formyl-methionyl peptides active at extremely low concentrations (e.g., 10−9-10−10 M) and referred to as formyl chemotactic peptides (fCP). Phagocytic cells contain specific receptors that bind the formyl Met-Leu-Phe (SEQ ID NO:1) peptide (fCP) with relatively high affinity, whereas the affinity of the desformyl derivative (dfCP) to the receptor is two orders of magnitude smaller (Freer et al., 1980, 1982). The fCP also induce degranulation of polymorphonuclear leukocytes (PMN) and cause the release of antimicrobial agents in the surrounding milieu (Becker, 1976; Niedel and Cuatrecasas, 1980). Most importantly, it has been shown that the formyl chemotactic peptide, when immobilized on particles, greatly enhances their phagocytosis by PMN (Becker, 1976).